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Research team claims fossil-only study of placental mammalian evolution time frame is wrong

Posted onJanuary 16, 2014

An artists rendering of the hypothetical placental ancestor, a small insect-eating animal. Credit: Carl BuellA team of researchers from the U.K. (led by Mario dos Reis) is directly challenging the results of a study conducted by another team (led by Maureen O’Leary) that concluded last year that placental mammals came to exist after the demise of the dinosaurs, not before. dos Reis et al maintain that their study using what’s known as the molecular clock, proves that placental mammals came before the demise of the dinosaurs. They have published a paper in the journal Biology Lettersdescribing how they came to their conclusions and why the other team is wrong.

Placental mammals are a kind of mammal, they deliver babies rather than eggs, and include all mammals except marsupials—that much biologists can agree on. When the first such mammals first evolved is still subject to fierce debate. The problem comes down to how those seeking to find the definitive answer go about it. O’Leary et al claim the only proven method of dating timelines for organisms is by studying the fossil evidence—no fossils of placental mammals have been found that predate the time when the dinosaurs died out. dos Reis and others on his side shoot holes in that line of reasoning by noting that it’s eminently possible that some organisms, such as placental mammals, existed for which there is no fossil record.